Apparatuses consistent with exemplary embodiment relate to a flocculation basin inclusion or exclusion type water treatment apparatus using dissolved air floatation, and more particularly, to a water treatment apparatus using dissolved air floatation which uses not the final treated water but the initial feed water to form saturated water to generate fine bubbles.
In general, a water treatment apparatus is an apparatus which produces fresh water using seawater to utilize the fresh water as water for living, eating or industrial use or properly treat raw water to purify waste water. Such a water treatment apparatus includes a device for removing foreign matters mixed in raw water.
As a unit process for removing foreign matters, there is dissolved air floatation (DAF). The dissolved air floatation is a method of mixing a coagulant to raw water to be treated to flocculate foreign matters contained in the raw water and raising the flocculate foreign matters together with fine bubbles to the surface to remove them, and is applied as a pretreatment process in the entire water treatment facilities or seawater desalination facilities.
Referring to FIG. 1, a conventional dissolved air floatation apparatus will be described in detail. When a coagulant is inserted into feed water to flocculate algae, suspended solids of low density or particulates, such as organic compounds, or floating materials contained in the feed water. In a flocculation basin 1, the feed water to which the coagulant is inserted is mixed through a stirrer and the mixture grows up to a floc size suitable for floatation.
“Floc” means a large mass that fine particles, such as suspended solids, organic matters and inorganic matters, contained in the raw water are flocculated by the coagulant, and generally means a collected thing formed by flocculation of particles of 0.1 μm or more. Flocs of small size or density which cannot be removed through filtration or precipitation are floated onto the surface of water to be removed using the dissolved air floatation.
The flocs generated and grown up in the flocculation basin 1 is transferred to a contact zone 2 and is floated to the surface of water by colliding and connecting with fine bubbles generated from the lower to be floated to the surface of water, and then, are removed through a scum removal device in a separation zone 3.
In the meantime, the conventional dissolved air floatation apparatus separates a recycling flow, which is some of treated water of the end product, by piping, sends the recycling flow to a saturator 7 through a pump 5, and saturates the recycling flow by supplying compressed air of 4 to 7 bars using an air compressor 6 so as to realize a rapid pressure drop through a nozzle 4 mounted at a lower part of the contact zone 2, such that fine bubbles are formed in the contact zone 2.
In general, a flow rate of the recycling flow separated from the treated water is about 10 to 20% of feed water, and can enhance performance of the apparatus because the quantity of generated fine bubbles increases when the flow rate of the recycling flow is increased. However, when the flow rate of the recycling flow is increased, the flow velocity in the separation zone 3 is increased and the flocculated bodies of the flocs and fine bubbles are not floated and are discharged to the following process.
The surface loading rate which is a performance index of the dissolved air floatation apparatus was lower than the theoretical rise speed of bubbles, which was 5 to 15 m/hr, but recently, there is a high rate DAF which has a surface loading rate of 20 to 40 m/hr. However, as described above, because the surface loading rate is faster than the rise speed of bubbles, if the floatation basin is designed in an error, contaminants and bubbles are discharged out together with the treated water, and hence, it may damage to the following process, and the actual surface loading rate may fall short of the surface loading rate because fine bubbles are generated using the recycling flow that the final treated water is separated by piping.